This invention relates to a closure device for a vacuum conveying unit which operates under a vacuum, i.e. under reduced pressure, and which has a circular outlet.
This type of a conveying unit is known, for example, from the brochure of Mann+Hummel GmbH, VKD 287.2 D, and is used for conveying plastic granules. This conveying unit operates by means of a suction blower. The plastic granules are conveyed into the unit from a container by means of the vacuum. After the expiration of a set conveying time, the suction blower will switch off. The material to be conveyed situated in the conveying unit presses open a flap, which is arranged over the conveyor outlet, and flows out of the conveying unit. After the material to be conveyed has flowed out, a counterweight urges the flap into the closed position and the suction operation starts again.
Published German Patent Application No. DE 3,406,824 discloses a bell-shaped valve for blocking an outlet. This bell-shaped valve is used particularly where very abrasive media are processed. So that no abrasion will occur on the bell-shaped valve and the other mechanical components of the valve, a protective sleeve is provided which rests against a guide body. Instead of rotary flaps or bell-shaped valves, ball valves or squeeze valves may also be used. These can be actuated electro-pneumatically.
A special problem presented by plastic processing machines is represented by cable sheathing extruders for producing cables of cross-linked polyethylene (PE). These cables are used exclusively in the high-voltage range of up to 440 KV. In this case, not only must the PE-insulating layer have a certain minimum thickness for the insulation, but its material must also be extremely pure. Because of a long-term effect, even traces of impurities of a magnitude of 40 .mu.m may impair the insulation of the cable and result in a breakdown. For this reason, all parts which come in contact with the cable sheathing material are without exception made of special steel with an electrolytically polished surface so that no abrasion or impurities can arrive in the insulating mass. Since this polyethylene is a cross-linkable material, one part peroxide, which is required for the cross-linking, is added to the polyethylene. This peroxide has the unpleasant characteristic that it separates from the granules and adheres to dead corners of an aggregate, ages there and, when detached in the extrudate, leads to a reduction in quality. For this reason, the aggregates for the transport and handling of this material, in addition to having an absolutely non-rusting, highly-polished surface, must be designed in such a shape that they have no dead corners.
Conventional closure devices for conveying units have not provided a satisfactory solution in this regard because any type of moved flap causes the slightest metallic abrasion and because it has not been possible no design a flap closure device without dead spaces.
Thus there has remained a need in the art for a closure device for a vacuum conveyor which would eliminate these disadvantages.